Fluorescence resonance energy transfer (FRET) can serve as a nanoscale molecular ruler. When used in imaging applications, it is a highly sensitive reporter of donor-acceptor molecular configuration. In most cases, FRET is utilized with standard one-photon excitation. Extensions of FRET to two-photon excitation are generally hampered by the problem of donor and acceptor bleed through, which imposes the requirement of technically complicated spectral unmixing algorithms or fluorescence lifetime measurements. We propose to considerably simplify the detection of FRET with two-photon excitation. Our goal is to establish the feasibility of modulating FRET signal using ultrasonic waves. Since FRET is sensitive to distance between donor-acceptor pairs at the nanoscale, mechanical compression and tension produced by sound waves are likely to modulate the FRET signal, serving as a useful signature to distinguish FRET from donor and acceptor bleed-through, which is a particular problem in multiphoton FRET. In particular, we propose to conduct preliminary experiments to establish the premise that ultrasound modulates FRET. We will investigate two types of samples: genetically encoded FRET probes in tissue (or tissue-like environments), and labeled microbubbles in solution. In the former case, we will investigate the application of our technique to in-vivo imaging in C.Elegans. FRET is a powerful tool for functional imaging. We believe that our hybrid ultrasound-optical technique will significantly improve the isolation of FRET signals and therefore be of benefit to any FRET-based functional imaging application. Moreover, when coupled with two-photon excitation, as proposed here, our technique should be well adapted to thick tissue imaging, thereby benefiting the in-vivo imaging community. PUBLIC HEALTH RELEVANCE: Fluorescence resonance energy transfer (FRET) is a powerful research technique to image cellular function in tissue. We propose to improve FRET signal generation with the use of ultrasound. This will benefit any FRET-based imaging applications, and will be specifically adapted to in-vivo microscopy in thick tissue.